Explain to her that if she has an eating disorder, that she is not only risking her life, but she is taking the risk that she might not be capable of having children as a result of the effects having an eating disorder would have on her body, which is a major reason that, for example, when I have a son, G-d willing, of shidduch dating age, I would specifically rule out anyone with the possibility of an eating disorder/ symptoms of an eating disorder for that reason. Additionally, if a woman has an eating disorder, and she does have a child, the child may pick up on the mother’s cues, chas ve’shalom, and decide that not eating is cool as well, and then end up in a bad situation.
This is not the right environment to put oneself into or risk putting future offspring into, so why should she do this to herself?